Air filter media produced by adding a binder to glass fibers and forming the mixture into a sheet have hitherto been used frequently for clean rooms. However, such filter media has some problems. For example, self-dusting is caused by the presence of adherent fiber debris in the filter media or by bending during processing, or if the amount of a binder is he increased to prevent self-dusting, an increase in pressure loss results (see JP-A-63-16019; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Furthermore, there has been another problem that if this filter media comes in contact with a kind of chemicals such as hydrofluoric acid, the glass and binder are deteriorated to thereby cause dusting.
An electric filter medium made of synthetic fibers has been proposed in order to eliminate these problems (see JP-A-54-53365), but such a filter medium has a problem of electric attenuation.
PTFE is a clean material with chemical resistance. Porous PTFE membranes have hence come to be used recently as filter media in various fields. In particular, an extremely thin porous PTFE membrane having substantially no large knots and consisting only of fibers is useful as an air filter medium for use in the fields where a high-degree of clean atmosphere is required, such as, e.g., the production of semiconductors.
Such useful porous PTFE membranes can be produced, for example, by forming a semi-sintered PTFE sheet (see JP-A-59-152825 which corresponds to U.S. Pat. No. 4,596,837) and then biaxially stretching the sheet to render it porous (see JP-A-3-221541 or JP-A-5-202217). However, this prior art process has a difficulty in fixing such conditions that the "semi-sintered state", which is suitable for the subsequent stretching, is realized. Especially because the range of the temperature condition is extremely narrow, it has been considerably difficult to industrially realize the desired semi-sintered state. For this reason, the above-described useful porous PTFE membrane could not be produced efficiently. The use of a porous PTFE membrane obtained by stretching a sheet-form unsintered PTFE molding as a high-performance air filter medium is frequently proposed, for example, in International Publication Wo 94/16802 and JP-A-7-196831.
The porous PTFE membranes proposed in International Publication WO 94/16802 and JP-A-7-196831 each has an extremely small pore diameter (average pore diameter) of 0.5 .mu.m or smaller so as to attain a heightened collection efficiency and has an extremely small thickness so as to retain a low pressure loss. However, the prior art PTFE membranes have the following problems because of the structure having a reduced pore diameter and a highly reduced thickness as described above. One problem is that a sufficiently high cleanliness cannot be provided, since the porous membranes have large fluctuations in thickness and pore diameter based on the respective average values thereof and hence cause leakage attributable to parts thereof having an extremely low pressure loss and to pinholes, that is, particles not smaller than those having a particle size to be filtered off pass through the membranes. Another problem is that since the fluctuations in thickness and pore diameter results in fluctuations in pressure loss, a filter unit employing the PTFE membrane exhibits a wide distribution of air flow rate and cannot exhibit a laminar air flow.
On the other hand, it is known that collection efficiency and pressure loss, which are the most important properties of filter media, are generally inconsistent with each other. Namely, increasing the efficiency of collection results in increased pressure losses. As a measure of a balance between collection efficiency and pressure loss, PF (performance of filter) value is frequently used. This PF value is defined by the following Equation (1); the higher the PF value of a filter medium, the higher the efficiency of dust collection of the filter medium and the lower the pressure loss thereof.